Photoconductor in a copolymer binder of vinyl acetate, vinyl laurate and an {60,{62 -ethylenically unsaturated acid

ABSTRACT

A photoconductive recording layer for use in electrophotography comprises a photoconductive pigment dispersed in an electrically insulating binder therefor consisting of a copolymer of 83 to 95.5 percent by weight of vinyl acetate, 4 to 15 percent by weight of vinyl laurate and 0.5 to 2 percent by weight of an Alpha , Beta -ethylenically unsaturated acid, preferably crotonic acid.

United States Patent Verhille et al.

PHOTOCONDUCTOR IN A COPOLYMER BINDER OF VINYL ACETATE, VINYL LAURATE AND AN a,B-ETHYLENICALLY UNSATURATED ACID Inventors: Karel Eugeen Verhille, Hoboken;

Luciaan Frans Voet, Katelijne-Waver, both of Belgium Assignee: Agfa-Gevaert N.V., Mortsel,

Belgium Filed: June 6, 1972 Appl. No.: 260,202

Foreign Application Priority Data June 8, 1971 Belgium 19477 us. (:1 96/1.5,96/l.2,96/1.6, 96/1.7,96/l.8, 117/161 UT, 260/41 B,

1m. (:1. G03g 5/04, G03g 5/06 Field of Search..... 96/1 PC, 1.5, 1.8; 260/808, 260/41 B Feb. 12, 1974 [56] References Cited UNITED STATES PATENTS 3,644,308 2/1972 Carpentier 260/41 B X 3,258,443 6/1966 Cantor 260/808 X 3,245,786 4/1966 Cassiers et a1.... 96/l.8 3,595,691 7/1971 Verhille et al.... 96/1 .5 X 3,160,503 12/1964 Cady 96/l.8

Primary ExaminerRoland E. Martin, Jr. Attorney, Agent, or Firm-William J. Daniel [57] ABSTRACT 8 Claims, N0 Drawings PI-IOTOCONDUCTOR IN A COPOLYMER BINDER OF VINYL ACETATE, VINYL LAURATE AND AN a,B-ETHYLENICALLY UNSATURATED ACID The invention relates to photoconductive recording layers for use in electrophotography and to processes of producing same.

An electrophotographic copying material consists of an electroconductive support having thereon a photoconductive recording layer formed of a photoconductive pigment dispersed in an insulating binder material. When such a material is used, an electrostatic charge is applied to the photoconductive recording layer, which is then exposed image-wise to light. At the places struck by light the electrostatic charge leaks away and the resulting latent electrostatic image is developed and thus made visible by treating with a finely divided toner material. The toner image can be fixed on the layer itself according to any known method such as heating or solvent fixing, or can be transferred to a receptor surface and fixed thereon.

According to the present invention there is provided a photoconductive recording layer for use in electrophotography, comprising a photoconductive pigment dispersed in an electrically insulating binder therefor, said binder consisting ofa copolymer of 83 to 95.5 percent by weight of vinyl acetate, 4 to 15 percent by weight of vinyl laurate and 0.5 to 2 percent by weight of an a, B-ethylenically unsaturated acid, preferably crotonic acid.

The invention also comprises a method for producing an electrophotographic recording element, comprising the steps of preparing a photoconductive pigmentbinder composition as hereinbefore defined, applying a solution of the photoconductive binder composition to an electroconductive support, and drying to remove volatile material therefrom.

Paper is normally used as support for the photoconductive recording layer, although other supports such as metal foils or sheets, glass, textile materials and plastic films can also be used. When the electroconductivity of the support is not high enough, the electroconductive properties of the support can be improved by the application thereto of a known conducting layer. Anyway, the resistivity of the support at a relative humidity of 50 percent has to be lower than approximately 1.10 Ohm/cm.

The photoconductive recording layer composition of the invention is applied to the support by spraying, painting, roller-application, dip-coating or wiping techniques. The coating is then normally dried at an elevated temperature. Preferably the coating mixture contains zinc oxide as photoconductive pigment in a weight ratio of 90 to 75 percent by weight with respect to the total solid content of the dried photoconductive layer. Good recording and reproduction results are obtained with photoconductive layers of a thickness between one and 20 micron, preferably between three and 10 micron. Layers that are too thin possess an insufficient insulating power, whereas layers that are too thick present undesirable mechanical and photographic characteristics. Instead of zince oxide other photoconductive oxides, sulphides, iodides and tellurides of zinc, cadmium, antimony, mercury, bismuth, titanium, thallium, lead, aluminium, silver and arsenic can also be used, either alone or in combination. Organic A mono-meric and polymeric photoconductors,

e.g. those described in the Belgian Patent Specification No. 587,300 and in the United KingdomPatent Specifications Nos. 964,871, 964,873, 964,874, 964,875, 964,876, 964,877, 964,879, 970,937, 980,879, 1,001,064, 1,004,461, and 1,006,877 can also be used.

The electrically insulating polymeric binder material for the photoconductive layer is a copolymer formed of vinyl acetate, vinyl laurate, and a a, B-ethylenically unsaturated acid used in the proportions as indicated. Although crotonic acid is preferred for the a, B-ethylenically unsaturated acid, other unsaturated carboxylic acids can also'be used, e.g. acrylic acid, methacrylic acid, maleic acid, itaconic acid, and citraconic acid.

When amounts of unsaturated acid larger than the 2 percent by weight indicated above are introduced into the copolymer the latter still shows very interesting properties as an insulating binding material for photoconductive pigments though it has a tendency of increased gellation with growing acid ratio, so that the conversion of a solution of the copolymer into a layer becomes almost impossible.

On the other hand, at least 0.5 percent by weight of unsaturated acid groups are required in the copolymer to ensure sufficient adsorption of the copolymer to the zinc oxide pigment particles. This adsorption is necessary to improve the photographic properties of the photoconductive layer at high relative humidities. Indeed, e.g. it is known that polystyrene, although being com pletely hydrophobic, is fully inappropriate as a binding material for zinc oxide. With polystyrene a rapid dark decay of the electric charges applied to the photoconductive layer is noted at high relative humidities. And this rapid dark decay is due to the absence of polar groups in the polymer.

Vinyl laurate is built in in the copolymer as an internal plasticizer. At least 4 percent by weight of vinyl laurate units have to be present in the copolymer to improve the plastic properties of the layer and to ensure a diminished curling of the zinc oxide layer even in extreme conditions of relative humidity. Further the vinyl laurate content of the copolymer has to be at most 15 percent by weight since it has been found that at higher relative humidities and for vinyl laurate contents above 15 percent the electrostatic chargeability rapidly di minishes. The electrostatic chargeability of the photoconductive layer is the charge, expressed in volts, taken up by the layer after charging with a coronadischarging apparatus having between the ground and the wires a potential difference of 6000 V. The voltage appearing at the electrometer terminals by passing thesamples directly after charging under the probe of a dynamic-capacitor electrometer as described by Giaimo, R.C.A. Review, 2 (1961) 780-790, is a measure of the charge density obtained.

Also for vinyl laurate contents above 15 percent by weight this increased dark decay is noticed at normal as well as at higher relative humidities and for copolymers of lower as well as of higher molecular weight. In these measurements the dark decay is defined as the loss of charge found on the layer after charging with the same corona-discharging apparatus followed by keeping the material for 30 seconds in the dark.

Another reason to keep the vinyl laurate content of the insulating copolymer binding material below 15 percent is the fact that beyond 15 percent the sensitiv ity of the material diminishes. This sensitivity of the photoconductive recording layer will be represented in the following examples with a relative number, 4 indicating a useful sensitivity for most of the electrophotographic apparatuses on the market, and 6 being found for a material, the sensitivity of which is rather too small.

The molecular weight of the copolymer is of minor importance. In our experiments we used copolymers having molecular weights between approximatively 30.000 and 80.000.

The photoconductive recording layers of the invention may contain, in addition to the photoconductive substance(s) and binder, optical sensitizers, e.g. those mentioned in United Kingdom Patent Specifications Nos. 1,020,504 and 1,020,506, additives known in coating techniques, eg pigments (see e.g. United Kingdom Patent Specification No. 1,007,349), compounds influencing the gloss and/or the viscosity, and compounds that counteract aging and/or oxidation of the layer, or that influence their thermal stability.

A very substantial increase in image density, which is probably due to an increase in sensitivity, can be obtained by the use, in the photoconductive recording layer, of substances increasing the dark-resistivity, e.g. those described'and claimed in the United Kingdom Patent Specification Nos. 1,020,504 and 1,020,506.

It was very interesting to find that the photoconductive layers of the invention could be used for the elec- Examples 1 to 3 The following mixture was ground in a sand-mill at a ratio of 60 litres/hour:

1,2-dichloroethunc 0.566 kg ethyl acetate 0.283 kg 20% solution of copolymer 1 kg photoconductive zinc oxide 1 kg solution of tetrachlorophthalic acid anhydride in ethanol 33 ml 1% solution of bromophenol in methanol '12 ml 2% solution of eosin in dimethylt'ormamide 0.6 ml 0.5% solution of fluoreseein in methanol 9.6 ml 2% solution of silicon oil in 1,2-dichloroethane 33 ml.

The copolymer of vinyl acetate, vinyl laurate, and crotonic acid used had a molecular weight of about 50.000 and possessed different vinyl laurate and crotonic acid contents as indicated in Table l. The copolymer was dissolved as a percent solution in a 65:35 by volume mixture of 1,2-dichloroethane and ethyl acetate.

The dispersion formed was applied to an electroconductive paper support at a ratio of g of zinc oxide per square meter, whereafter the material was dried and kept in the dark for 24 hours.

Samples of the materials formed were charged negatively as indicated above and the chargeability and dark decay were measured at relative humidities (R.H.) of 50 and 70 percent. The sensitivity of the materials was trophotographic reproduction of continuous tone iml oted,

Table 1 Example Copolymer Sensi- Chargeability dark decay no. tivity in Volts vinyl crotonic at 50 R.H. at 70 R.H., laurate acid content content A series of comparisons were made with copolymers having also a molecular weight of about 50.000, having varying crotonic acid content, but having a vinyl laurate content of 20 percent, thus outside the scope of out invention. The following results were found.

When the figures of table 2 were compared with those of Table 1 it appeared that for vinyl laurate contents above 15 percent, even though the chargeability was about the same, the dark decay, was far too high, especially at higher relative humidities. Further the sensitivity also tended to diminish as indicated by the number 6 for a copolymer having a vinyl laurate content of 20 percent and a crotonic acid content of 1.5 percent.

The figures found for the copolymers of table 1 were also compared with those of other copolymers of varying molecular weight as indicated in table 3.

The copolymers of vinyl acetate, vinyl laurate. and crotonic acid had a molecular weight of approximatively 50,000 and were composed as indicated in Table 4.

Table 3 Chargeability Comparison Copolymer Sensidark decay in no. tivity Volts vinyl crotonic molecular at 50 at 70 laurate acid weight R.H. R.H. content content Table 4 Example Copolymer Sensi- Chargeability dark decay no. tivity in Volts vinyl crotonic at 50% RH. at 70% R.H. laurate acid content content Here also the dark decay was too high. Moreover, the sensitivity was insufficient as indicated by the numbers 5 and 6.

EXAMPLE 4 The following mixture was ground in a sand-mill at a ratio of 60 litres per hour:

1,2-dichloroethane 1.500 kg ethyl acetate 0.476 kg copolymer 0.160 kg photoconductive zinc oxide 1 kg 10 solution of tetrachlnrophthalic acid anhydride in ethanol 33 ml 2 solution of silicon oil in 1,2-dichloroethane ml 2 ll; solution of eosin in dimethyl- 0.6 ml formamide 0.5 solution of fluorescein 9.6 ml in methanol The copolymer used had a molecular weight of approximatively 50.000 and contained 90.5 percent of vinyl acetate, 8 percent of vinyl laurate, and 1.5 percent of crotonic acid.

The sensitivity of the electrophotographic material showed to be 4 and the chargeability-dark decay at relative humidities of 50 and 70 percent respectively 430-370 and 430-250 Volts.

EXAMPLES 5 AND 6 The following mixture was ground in a sand-mill at a ratio of 60 litres per hour 1,2-dichloroethane 1.490 kg butyl acetate 0.264 kg copolymer Example 5 0.180 kg Example 6 0.160 kg 10 solution of tetrachlorophthalic acid in ethanol '33 ml photoconductive zinc oxide 1 kg 2 solution of cosin in dimethylt'ormamide 0.6 ml

0.5 solution of fluorescein in methanol 9.6 ml

In both cases the dark decay showed to be very low and the sensitivity was very good as appears from the values 4 to 3.

EXAMPLE 7 The coating was carried out in such a way that the dried layer contained 2 g of gelatin per sq.m. The electrical resistivity of the coating was 1 X 10 Ohm per sq.cm.

An electrophotographic recording material was prepared by coating onto said conductive layer a solution containing an organic photoconductor having the following structural formula:

(melting polntz1l2 C.)

Rhodamine B (C.l. Basic Violet l; C.l. 45,l70) 0.020 g copolymer of vinyl acetate, vinyl Iaurate and crotonic acid (90.5:8:l.5 g methylene chloride 100 ml The solution was applied in such a way that the dried recording layer contained 3 g per sq.m. of said quinolin-2-0ne compound as photoconductor.

After a negative corona charging with a potential difference of 6000 V between the corona wires and the earth, the charged recording layer was contact-exposed for 1 see. through a positive transparency of a test chart with a tungsten filament lamp exposing the recording layer with 2400 lux and having a colour temperature of 2600K.

After the exposure the development was carried out by means of an electrophoretic developer obtained by diluting the concentrated developer composition described hereinafter in a volume ratio of /1000 with of lSOPAR H (trade name for an isoparaffinic hydrocarbon mixture having'a boiling range of l77l88C sold by Esso Belgium, N.V., Antwerp, Belgium):

carbon black (average particle size:

nm) 30 g zinc monotridecyl phosphate as dispersing nt l.5 g lSOPAR H (trade name) 750 ml resin solution prepared as described hereinafter I50 g The resin binder solution was prepared by heating 500 g of ALKYDAL L 67 (trade name of Farbenfabriken Bayer A.G., Leverkusen, W. Germany, for an alkyd resin modified with linseed oil (67 percent by weight) and 500 ml of white spirit containing 11 percent by weight of aromatic compounds, up to 60C till a clear solution was obtained, and subsequent cooling.

EXAMPLE 8 A photoconductive recording layer as described in Example 5 was applied to an electronconductive paper support. The photoconductive recording layer was charged negatively and contact-exposed for seconds. As original a colour transparency was used. During a first exposure a green filter was laid between the original and the photoconductive recording layer, said filter transmitting only light of wavelengths around 535 nm. The latent image formed was developed with-a known electrophoretic development solution containing a magenta dye-stuff. A positive magenta separation image was obtained from the original.

The so-called electrophoretic development was described by Metcalf Liquid Developers for Xerography, Journal of Scientific Instruments 32 (1955) 74-75 and in the United Kingdom Patent Specification No. 835,044 and United States Patent Specification lmmediately thereafter the photoconductive recording layer already carrying the magenta image was charged again and image-wise exposed in register for 45 seconds to the same colour transparency with the aid of a filter transmitting only light of wavelengths around 460 nm. The new latent image was developed again, this time with an electrophoretic development solution containing a yellow dyestuff.

Once more the photoconductive recording layer was charged and exposed in register for 10 seconds to the colour transparency through a filter transmitting only light of,wavelengths around 6l5 nm. An electrophoretic development solution containing a cyan dyestuff was used.

A multicoloured copy was obtained from the original.

What we claim is:

l. A photoconductive recording material for use in electrophotography, comprising a conductive support and a photoconductive layer thereon comprising a photoconductive pigment dispersed in an electrically insulating binder, said binder consisting of a copolymer of 83 to 95.5percent by weight of vinyl acetate, 4 to l5 percent by weight of vinyl laurate, and 0.5 to 2 percent by weight of an a, ,B-ethylenically unsaturated acid.

2. A photoconductive recording material according to claim ll, wherein the a, B-ethylenically unsaturated acid is crotonic acid.

3. A photoconductive recording layer according to claim 1, wherein the binder is a copolymer of vinyl acetate, vinyl laurate, and crotonic acid (90.5:8:l.5 percent by weight).

4. A photoconductive recording material according to any of claim 1, wherein the phtoconductive pigment is photoconductive zinc oxide.

5. A photoconductive recording material according to claim 1, wherein the photoconductive pigment is an organic photoconductor.

6. A photoconductive recording material according to claim 5, wherein the photoconductive pigment is a compound according to the formula:

7. A photoconductive recording material according to claim 1, wherein the photoconductive pigment constitutes 90 to percent by weight of the total solid contents of the dried photoconductive layer.

8. A photoconductive recording material according to claim 1, wherein the photoconductive layer has a thickness between three and 10 micron.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,791,825 Dated February 1974 Inventor(s) Karel Eugeen Verhille et al Itis certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the Patent, under "Foreign Application Priority data", "Belgium" should read Great Britain Column 8, Claim 3, Line 27; "layer" should read material q Signed and Scaled this Twenty-second D3) 0f March 1977 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer COMMIT-Hi0"?! nj'Palems and Trademarks 

2. A photoconductive recording material according to claim 1, wherein the Alpha , Beta -ethylenically unsaturated acid is crotonic acid.
 3. A photoconductive recording layer according to claim 1, wherein the binder is a copolymer of vinyl acetate, vinyl laurate, and crotonic acid (90.5:8:1.5 percent by weight).
 4. A photoconductive recording material according to any of claim 1, wherein the phtoconductive pigment is photoconductive zinc oxide.
 5. A photoconductive recording material according to claim 1, wherein the photoconductive pigment is an organic photoconductor.
 6. A photoconductive recording material according to claim 5, wherein the photoconductive pigment is a compound according to the formula:
 7. A photoconductive recording material according to claim 1, wherein the photoconductive pigment constitutes 90 to 75 percent by weight of the total solid contents of the dried photoconductive layer.
 8. A photoconductive recording material according to claim 1, wherein the photoconductive layer has a thickness between three and 10 micron. 